Place the 2 Tablespoons of butter in an 8 by 8 pan and set it in the oven to melt while you whip up the ingredients. Once the butter has melted, be careful not to let it sit too long in the oven by itself or it can burn. This usually isn't an issue if you quickly whip up the other ingredients.
To a blender add the eggs, milk, vanilla, flour, cocoa powder, sugar and salt. Blend together about 20 seconds until fully combined.
Pour the blended mixture into the hot pan with melted butter and place in the oven.
Cocoa Powder: This recipe tastes best and has a more intense chocolate flavor with a rich dark cocoa powder like Hershey's special dark chocolate cocoa powder (affiliate link). You can substitute regular cocoa powder and it will still taste delicious, just not quite as "chocolatey" in flavor.